This relates generally to imaging devices, and more particularly, to CMOS image sensors.
Modern electronic devices such cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel may include a photosensor such as a photodiode that receives incident photons (light) and converts the photons into electrical signals. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format or any other suitable image format.
In certain applications, it may be desirable to capture high-dynamic range images. While highlight and shadow detail may be lost using a conventional image sensor, highlight and shadow detail may be retained using image sensors with high-dynamic-range (HDR) imaging capabilities.
Two major conventional approaches for achieving higher dynamic range in image sensors include: (1) an in-pixel HDR implementation based on companding, multiple storages, and signal controlled reset and (2) dual image captures with different integration times. Drawbacks to the companding pixel approach include nonlinear output that hampers subsequent color processing, increased pixel fixed-pattern noise (FPN), signal-to-noise (SNR) dip at knee point, and low contrast in the highlights. The multiple storage and signal controlled reset approaches are undesirable because they require extra pixel area along with additional column circuitry. On the other hand, dual image capture can also suffer from SNR dip at knee point and may require high speed non-destructive readout along with on-chip memory and additional column circuitry.
It would therefore be desirable to be able to provide improved imaging devices for capturing high-dynamic-range images.